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What results can you expect from soil-residual herbicides?

I hope my last article and my continued remarks regarding soil residual herbicides will be taken in the positive vein in which they are intended. I am all about helping get more soil-applied herbicides used in Roundup Ready, LibertyLink and conventional soybean production systems.

At the same time, growers should be realistic about what to expect from them. Do not lose sight of the fact that if soil residual herbicides were the answer to most of our weed problems, Roundup Ready would not have taken the market by storm in the mid 1990s.

For the most part we have the same residual herbicides now that we had then.

Where conventional tillage will be used for seedbed preparation, preplant incorporated herbicides can provide a lot of annual grass, red rice and small-seeded broadleaf weed control for the money. When a herbicide is incorporated into a moist clod-free seedbed, rainfall for activation is not as critical as it is with a pre-emergence treatment.

While some growers continue to use incorporated treatments successfully, I doubt we will see a large increase in usage. They obviously can not be used where the seedbed will not be tilled prior to planting and that term “clod-free” means they do not work well on clay soils.

If there is going to be a big increase in the use of residual herbicides, it will likely be in surface-applied herbicides. If the herbicide is applied pre-emergence — after planting but before soybean and weed emergence — it must be activated by rainfall or overhead irrigation before the first flush of weeds occur to be effective.

The first question someone usually asks about a pre-emergence herbicide is, “How long will it wait on a rain?” Some herbicides degrade faster prior to activating rainfall than others, but this is not the main issue. If the herbicide remains on the surface waiting on a rain, it may provide some residual control following activation. However, these herbicides do not have much activity (“reach back” as it is often called) on weeds after they have emerged.

In Arkansas, weeds will often emerge in three to five days after planting and our rainfall frequency sure is not three to five days in the summer. For this reason, pre-emergence herbicides have always been erratic and will continue to be.

Because pre-emergence herbicides are erratic, I believe they have a much better fit in programs to help prevent herbicide resistance than to fight it after you have it.

There is a lot of interest in Prefix, for example, as a pre-emergence herbicide for Palmer pigweed resistance management in Roundup Ready soybeans. If you do not yet have resistant pigweed, Prefix can make a nice addition to the program because it has good pigweed activity if you get it activated timely. On the other hand, if you already have glyphosate-resistant pigweed and you are using Prefix or some other pre-emergence treatment to make your Roundup Ready program successful, you are immediately in the ditch if the first flush of pigweed emerges before activation.

My recommendation is to be very cautious where you use them if the success of your weed control program is entirely dependent on the pre-emergence treatment.

There is another approach you can take with surface-applied herbicides that may either improve consistency or at least let you know where you stand at planting. That is to apply the residual herbicide 10 to 14 days prior to planting — usually in a burn-down treatment.

Valor, for example, is the surface-applied herbicide that probably has the best pigweed activity. If you apply Valor a couple of weeks prior to planting, that is two additional weeks you have to get it activated. If you plant into a treatment that is already activated, it may be more consistent. While Valor can also be used at planting, the soybean injury potential is reduced considerably with the preplant treatment.

I think the preplant surface treatments will gain in popularity. I believe, however, they have a much better fit for prevention rather than control.

TAGS: Management
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